An encryption key could be stored in a QR code and the user could be sheltered from any technical issues.

To be honest if we’re talking truly accessible, even usernames and passwords aren’t great. Users forget them. That’s why a lot of sites these days offer to email you “magic links” to log you in. And if you want to do that you need to make sure you’re running a server sending emails that won’t immediately go to spam.

At a certain point if you want a truly scalable, decentralized platform you’re going to have to cut back on backend responsibilities. This is a logical answer to the problem.